There are many different types of pressure sensors used to measure pressures created by various forces in many different locations. Typically, pressure sensors are comprised of a strain gauge placed on a surface that deforms when a pressure is applied to the surface. Signals from the strain gauge are used to calculate the pressure applied to the surface by detecting the deformation of the surface due to the pressure. A strain gauge often takes the form of a variable resistor. These strain gauges are all very accurate but generally require calibration and maintenance in order to provide accurate pressure detection. Generally, in measuring a fluid flow, a pressure sensor is placed on either side of a nozzle formed within a conduit. The nozzle forms a restriction in the conduit forming a venturi and due to the Bernoulli effect, will result in different pressures on either side of the nozzle depending upon the flow characteristics within the conduit. Accordingly, by measuring the pressure on either side of the nozzle, the fluid flow or changes in the fluid flow can be calculated and determined. The pressure drop across an orifice is well understood and directly proportional to fluid flow. The fluid flow is proportional to the square root of the pressure differential. Very small changes in fluid flow may be detected. There are different devices that have been developed for detecting fluid flow in different applications. One such application is in the medical field where small fluid flows or changes in fluid flows must be accurately detected.
One such device is illustrated schematically in FIG. 1. FIG. 1 generally illustrates a fluid control system 10 having a control unit 12 and a replaceable or disposable unit 14. A replaceable conduit 16 formed in the replaceable or disposable unit 14 has an orifice or a nozzle 20. On either side of the orifice or nozzle 20 are flexible membranes 18 and 22. Inlet 24 permits fluid to flow in the direction of the arrows. The replaceable or disposable unit 14 is placed within the control unit 12. The inlet 24 then is coupled with the control unit conduit 30 having input 32. The flexible membranes 18 and 20 are then placed adjacent the first pressure sensor 28 and the second pressure sensor 26. The first and second pressure sensors 26 and 28 provide a first pressure signal 36 and a second pressure signal 34, respectively. These pressure signals 34 and 36 are transmitted to a flow controller 38 that controls a pump 40 coupled to the control unit conduit 30. Accordingly, depending upon the fluid pressure detected by the second and first pressure sensors 26 and 28, the fluid flow may be calculated and any changes detected.
FIG. 2 is a simplified schematic diagram illustrating the detection principal implemented in the device illustrated in FIG. 1. In FIG. 2, conduit 16′ contains a fluid flowing in the direction of the arrows. Orifice or nozzle 20′ creates a restriction and results in a pressure P1 being formed on one side of the orifice or nozzle 20′ and a pressure P2 formed on the other side of the orifice or nozzle 20′. A first pressure sensor 28′ is placed adjacent a flexible membrane 22′ on one side of the nozzle 20′ to detect the first pressure P1. Similarly, a second pressure sensor 26′ is placed adjacent a flexible membrane 18′ to detect pressure P2 on the other side of the orifice or nozzle 20′. Accordingly, the two pressure readings are used to determine fluid flow within the conduit 16′. While the fluid control and detection system illustrated in FIGS. 1 and 2 have been adequate to detect fluid flow in many applications, it requires the expense of two separate pressure sensors and the need to calibrate each of the two different pressure sensors so as to provide accurate flow detection. The use of two separate pressure sensors adds to the expense of the device as well as makes necessary the requirement for accurately calibrating each separate pressure sensor so as to assure accurate detection of flow. This added to the need for increased maintenance as well as possible errors in the flow rate detected due to errors in the calibration of the two different pressure sensors.
Therefore, there is a need to improve fluid flow control and detection systems so that they will be less expensive, more easily maintained, and provide more accurate results.